There are many varieties of U-Joints, some of which are incredibly complex. The easiest category referred to as Cardan U-Joints, happen 
to be either block-and-pin or bearing-and-cross types.
U-joints can be found with two hub models solid and bored. Sound hubs don’t have a machined hole. Bored hubs possess a hole and are named for the hole condition; round, hex, or square style. Two bored styles that deviate from these common shapes are splined, that have longitudinal grooves in the bore; and keyed, which have keyways to avoid rotation of the U-joint on the matching shaft.
Using the incorrect lube can lead to burned trunnions.
Unless in any other case recommended, use a high quality E.P. (serious pressure) grease to provider most vehicular, professional and auxiliary drive shaft applications.
Mechanically flexible U-Joints accommodate end movement by simply using a telescoping shaft (sq . shafting or splines). U-Joints function by a sliding motion between two flanges that are fork-shaped (a yoke) and having a hole (vision) radially through the attention that is linked by a cross. They allow larger angles than flexible couplings and are used in applications where great misalignment has to be accommodated (1 to 30 degrees).
Always make sure new, fresh grease is evident for all four U-joint seals.
Can be caused by operating angles which are too big.
Can be the effect of a bent or sprung yoke.
Overloading a drive shaft could cause yoke ears to bend. Bearings will not roll in the bearing cap if the yoke ears aren’t aligned. If the bearings stop rolling, they stay stationary and will “beat themselves” in to the area of the cross.
A “frozen” slip assembly won’t allow the travel shaft to lengthen or shorten. Every time the drive shaft tries to U Joint china shorten, the strain will be transmitted in to the bearings and they’ll indicate the cross trunnion. Unlike brinnell marks due to torque, brinnell marks that will be caused by a frozen slide are usually evident on the front and back areas of the cross trunnion.
Improper torque in U-bolt nuts could cause brinelling.
Most makers publish the recommended torque for a U-bolt nut.
Improper lube procedures, where recommended purging isn’t accomplished, can cause one or more bearings to be starved for grease.